This invention is directed to a method for preparing anhydrous nitric acid. More particularly, this invention is directed to a method for preparing anhydrous nitric acid by extracting an aqueous mixture of nitric acid and sulfuric acid with methylene chloride. The anhydrous nitric acid can be used as a solution in methylene chloride or isolated from the methylene chloride by distillation or crystallization.
Nitric acid is generally produced by the standard ammonia oxidation process as an aqueous solution in a concentration of from about 50 to about 70 percent by weight. Concentrations in this range are generally suitable for producting inorganic compounds such as ammonium nitrate and the like. However, for organic nitrations and many other uses anhydrous nitric acid is required. Since nitric acid forms an azeotrope with water at a concentration of about 68 percent by weight, the water cannot be separated by simple distillation. Heretofore, two methods have been commercially employed for making anhydrous nitric acid. These methods are extractive distillation and reaction with additional nitrogen oxides.
The most common method for producing anhydrous nitric acid (98-100 percent nitric acid) comprises distillation of a weak nitric acid with concentrated sulfuric acid, the sulfuric acid serving effectively as a dehydrating agent. In the Chemico concentration process 60 percent nitric acid is mixed with 93 percent sulfuric acid in a Raschig ring packed dehydrating tower provided with a steam-heated reboiler. The nitric acid vapor is distilled overhead and condensed while the sulfuric acid and water leave the bottom at a concentration of about 70 percent sulfuric acid. Water is then removed from the sulfuric acid in a concentrator and the sulfuric acid is recycled to the process.
An alternative to sulfuric acid which has found more recent application is a 72 percent solution of magnesium nitrate in water. The nitrate solution typically leaves the distillation column at a concentration of about 68 percent and is reconcentrated by flashing to a steam heated vacuum drum. The use of magnesium nitrate has been found to be more economical in smaller plants, particularly where existing sulfuric acid concentrating facilities are not available or where a sulfate free nitric acid is required.
Another method for producing anhydrous nitric acid which has gained widespread use, particularly in Europe, is the reaction with nitrogen oxides. Nitrogen tetroxide is separated from the process gases leaving the ammonia converter by refrigeration or by absorption in concentrated nitric acid. The tetroxide is then autoclaved with weak nitric acid and oxygen to yield a 98 percent product.
While the above mentioned processes are suitable for producing 98 percent plus nitric acid there is a need for improved processes which will produce anhydrous nitric acid. Therefore, it is the object of this invention to provide a new process for preparing anhydrous nitric acid.
Anhydrous perchloric acid has been prepared by extraction of a perchloric acid-oleum solution with methylene chloride. See the article by P. H. Plesch et al., "Chemistry and Industry," September, 1971, pp. 1043 and 1044.